Here is the Complete Procedure:
Examine The Wood You Are Using For The Sides
Look for any end checking. This may ruin the whole piece if these checks are long and deep enough. Very shallow checks or checks that are short should not cause a concern as the sides are often trimmed enough during the butt trimming and neck routing operations. Thickness sand the sides to just a whisker over what their final thickness will be. This makes the side wood easier to bend, and saves you a bunch of time when finish sanding your guitar.
Try to use straight grained, quarter-sawn wood for the guitar sides. As you gain experience in working with and bending wood you will be able to graduate to the more figured woods which are not necessarily quater-sawn or straight grained in nature.
Before you use highly figured woods such as Bubinga with a waterfall grain, or some of the pleated or curly woods like Curly Maple or Curly Mahogany, you should have a bit of experience under your belt. One way to get this experience is to purchase or make a a few practice sides from inexpensive wood to try out your bending methods without sacrificing hundreds of dollars worth of exotic wood.
Sand Wood Sides to Thickness – General Recommendations:
- Classical Guitars – 1.8mm (Final Thickness – Allow for some sanding after bending)
- Acoustic Guitars – 2.0mm (Final Thickness – Allow for some sanding after bending)
- Ukulele Sides – 1.5mm (Final Thickness – Allow for some sanding after bending)
Mark The Sides For Registration
First, it is important that you lay the sides on a workbench in book-matched fashion and fully understand how the grain patterns will look on the final product. If you are using a dark-colored wood, mark the center of the waist with a white pencil. Also, the waist centerline will have to show up very clearly on the side of the wood. The best way to do this is to place a small piece of masking tape over the edge at the waistline.
Then take a waterproof Sharpie Dry-marker and clearly mark the waist with a registration mark. In order to give you a little play while you are bending, make the waist registration line and edge mark about 1/2” (12mm) further from the butt end of the side. This will allow for a little movement which is usually the case when bending wood.
In order to keep the sides book-matched the registration marks should be on only one side of the wood. This will keep the sides book-matched – be sure to double-check this before you bend the second side so you don’t wind up with 2 left hand or 2 right handed sides. Not a very happy situation!
I also like to mark out the centerline of the waist on the stainless steel slats so registration of the slats and sides is very easy and any slippage is easily spotted. Again, the best method for doing this is with a water-proof Sharpie Dry-marker.
Prepare The Machine For Bending
Have plenty of work area around the side bender, and the work area is free of clutter and that you are working at a comfortable work height.
Preparing the sides for bending is very subjective, depending on who you talk to. Most luthiers wet the sides with a spray attachment on a bottle filled with distilled water, soaking them quite well, especially at the waist and bout curves. This is the only method that I utilize.
Other builders soak the sides in a tank of water for a few minutes and then immediately load them in a pre-heated bender. The objective here is to create steam when heat is applied to the wood, which makes the wood elastic and allows it to bend very easily around the form.
Thoroughly wet down the side wood with a spray mister, concentrating on the waist, and the upper bout curvatures. I like to spray both sides of the wood side with a spray bottle, then let the side set about ten minutes before spraying it one more time. This allows the side to absorb some of the moisture and aids in the steam generation to bend the side more easily.
Assemble the Sandwich
Get the sandwich situated first. Begin by laying the bottom stainless steel slat on your work bench. Next place the wet wood side over the slat while aligning the wood with the stainless steel waist registration markings. The heating blanket should be placed so it is in contact with the wood. Finally, follow-up with the top stainless steel slat. Place a spring clamp at the lower bout end of the sandwich to hold everything together.
Anchor The Bender To Your Workbench
Lay down the bottom SS slat first,then the wood side to be bent, the heating blanket and finally the top stainless steel slat. Align the marker on the waist with the stainless steel slat.
Before using the bending machine, it is best if the feet of the bender are anchored to the workbench. I use a couple of quick-grip clamps to do this. An alternative is to place the bender in a woodworkers vice, but this requires the removal of the feet. Another option is to clamp in on the work bench with the vice and bench dogs. Whichever method is used, assure that the bending machine is secured and will not move. You will be working with extremely hot stainless steel and any slip of the bender can cause very severe burns.
Prepare the Bending Machine For The Bending Mode
Load the proper form insert into the machine and install the waist guide bar to lock the form insert into the bending machine. Crank the waist retainer up as far as it will go. Insert the sandwich into the bender and slowly crank the press screw down to squeeze the sandwich about half-way to the waist. The sandwich still needs be able to move for fine-tuning adjustment and registration.
Look through the guide bar slot on the bender and try to spot the registration mark you made on the side. Adjust the sandwich until the registration mark is centered on the channel. Hook up the lower bout retainer with both extension springs. Do the same with the upper bout retainer. Next, hook up the waist retainer extension springs to the guide bar and crank the press screw until the guide bar just barely engages the form insert receiving channels (about 1/8” or 3mm).
Slide the lower bout retainer about half-way around the form insert or until it stays in place without sliding back. Adjust the spring clamp at the back of the sandwich so the sandwich stays in good contact with the heating blanket. This will provide even and thorough heating throughout the entire wood side.
Now Move to the Upper Bout
Install a couple of spring clamps on the sandwich to provide good contact between the heating blanket and the wood side material.out. Place a spring clamp on either side of the sandwich so is has good contact with the heating blanket here as well. The bender is now ready for the next step – the heating mode. If the spring clamps have rubber or plastic jaws, protect them from the heat by using a thin wood caul at the top and bottom of the stainless steel.
Take the thermocouple probe end and insert it between the heating blanket and the wood side. This can be anywhere along the sandwich you can find an access space. Once you start the bender up you need to have good contact by the probe to accurately monitor the temperature.
Prepare Yourself Prior To Starting The Bender
Once the bending machine reaches its high temperature, the side, the heating blanket and especially the stainless steel sheets can be as hot as 350 degrees F. This means you can be seriously burned without any protective wear. Be sure to wear leather gloves on both hands and a long-sleeved shirt prior to plugging the bender into the electrical receptacle.
Placement of the thermocouple probe between the heating blanket and the side wood.
Plug the heating blanket into the bender receptacle located on the back of the bending machine. Plug the bending machine into a 110v (or 220v) electrical receptacle and adjust the set point to 140 degrees centigrade on the digital thermostatic controller. Also be sure to start a timer at the same time. I prefer to use my iPhone timer for this and lay it on the workbench and set the alarm function for 15 minutes.
(Note that our Temperature Controller will handle any voltage up 240v. The only difference is the type of wall receptacle to be used. The heating blankets, while they look similar are made for either 120v or 220v service. They are clearly marked on the surface of the blanket)
Now you need to wait until the temperature reaches between 110 to 120 degrees C. With the 2.5 watt per square inch heating blanket this should take between 6 and 8 minutes. Monitor both the timer and the digital temperature controller. Adjust the thermocouple probe if needed to get a more accurate temperature readout. Look at the sandwich and you should start to see steam rising from the side cooking under the heating blanket. You will also be able to hear the side starting to boil. This is a good thing as it means the wood is prime for some easy bending.
Remove the spring clamp on the lower bout sandwich and slide the retainer all the way to the bottom of the form insert, or even a little bit past the bottom of the insert. This should pose no problems for you at all. Just use a slow steady pull on the retainer to bring it to its final placement. As you always should do when working with guitar sides – listen for any cracking or popping sounds as this is an indication of the wood fibers failing.
Next, remove the spring clamps from the upper bout sandwich. Slowly pull this retainer over the sandwich and around the form insert as well. This is the location where splitting of the wood is most likely to occur, as for most guitar shapes this is a very sharp bend. I usually pull the retainer down about half-way around the form insert and let it set for about a minute and then continue all the way to the end of the form insert.
Finally crank down the waist tension block with the press screw until the sandwich firmly contacts the upper and lower bouts of the form and the waist tension block is pressing tightly against the form insert at the waist.
Upon finishing all the bending operations, wait until the alarm on the timer goes off (this should be between 12 and 15 minutes after plugging in the bending machine. Simple disconnect the bender from the electrical source and let the side complete cool down.
I prefer to keep the side in the bender for between 3 and 4 hours before removal. Once you remove it, immediately place the side into your inside form and install the spreader jigs to hold it firmly against the form sides.
GLS Tip #1: If you find you have some resistance during side bending or hear cracking, popping, or other odd noise that possibly means the side is fracturing…leave the side sitting in the form for another 10 minutes to heat-up. Then try to finish the bend again. If you still have trouble, remove the retaining block and spray water on the side by separating the sandwich. Next place the sandwich together again and let it set. Now you should be able to easily bend the side.
GLS Tip #2: If, after removing the sides from the mold, there is some spring-back, you will need to fit the sides in a squeeze jig. This allows you to over-bend the sides so they will form more closely to the inside mold. Another option to touching up the sides is to finish bending the side on the hot pipe. Review this procedure by reading our article Side Bending – The Hot Iron.
GLS Tip #3: Sometimes you will find that the extension springs will not have enough power to pull the sides tightly against the form insert, especially at the bottom. The most effective way to correct for this condition is to install a bar clamp across the the form insert to pull the sides tightly in place.
GLS Tip #4: Temperature and Bending: This is a subject that is discussed quite a bit in guitar-building circles. Many luthiers claim that the wood has to reach 350 degrees F. before effective bending can take place. In my own experience, I have found that somewhat lower temperatures produce better results.
I have found that a maximum of 140 degrees C works perfect. With higher temperatures it is very easy to scorch or even burn the wood and once it is burned, or even scorched, it is ruined. At lower temperatures the wood still bends very easily and the possibility of scorching or burning the wood is almost completely eliminated. It’s best to do your own experimenting (with cheap wood) to find out what works best for you!
Using The Bending Machine With Light Bulbs (Guitar Bender Only)
While this is not the preferable option to obtain optimal results, it may be your only option if you are on a tight budget or only occasionally make guitars. If this is the case, you can follow the directions as we talked about earlier in this chapter with the following changes:
It is best to use a surface mount thermometer which can be attached to the top stainless steel slat to monitor your temperature progress. You will have to plug in the bending machine and let the sandwich heat for about 15 minutes or so while keeping a close eye on the thermometer. Once the temperature reaches about 120 to 130 degrees C, you can begin the bending process as before.
Again, keep the bender hot for at least 5 to 10 minutes at high temperature and then you can unplug the bender. Let the side cool down as before.
This method will take more experimentation on your behalf, but the results will be just as good as with the heating blanket, you just lack the speed and control when using light bulbs.
Preparing Sides After Bending
After the guitar sides have been bent they need to be placed into the inside form immediately to prevent spring-back. This is best done with two or three Spreader Jacks. Use one at the upper and lower bouts and one at the waist. Apply only enough pressure to have each side conform to the inside form. Using more than that will risk bending your inside form out of shape. Also, be sure to orient the sides so the top plate side is facing toward the floor of the form.
Next, it is time to cut through the butt joint and the neck block joint of the sides with a band saw or a Japanese Hand saw.
Clamp the sides in place in the inside form prior to cutting each joint, and relieve the tension on the spreader jacks so as to allow the inside form to remain in shape. Carefully cut through the butt joint with the saw, remove the scrap and butt both sides together. Make the same cut at the neck joint using a similar technique.
Replace the inside form restraints that bridge across the saw joints to maintain the inside form integrity. Now the sides should fit very nicely within the inside form. Place the spreader jacks in place to again maintain the guitar shape as close to the inside form as possible, even though it may seem as though they are not necessary.
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